There are several different techniques for producing a lightguide fiber for use in communications. One such technique comprises directing a constantly moving stream of reactants and oxygen through a glass substrate tube having a generally circular cross-section. The oxygen stream also will carry dopants to produce the appropriate or desired index of refraction in the finished lightguide fiber. The substrate glass is heated to a reaction temperature within a moving hot zone that traverses the outside of the tube, and the consequent reaction products and dopants are fused into a continuous layer on the inner wall of the tube. The resulting tube is referred to as a preform tube.
A torch assembly for heating a glass substrate tube to facilitate deposition is described in U.S. Pat. No. 4,231,777 which issued on Nov. 4, 1980, in the names of B. Lynch and F. P. Partus. A plurality of nozzles which are disposed radially of a rotatably supported glass substrate tube open to an arcuate surface of a housing that is mounted on a carriage and that is spaced a predetermined distance from the tube to be heated. Initially, one end of the tube is supported in the headstock of a lathe and the other end is welded to an exhaust tube that is supported in the tailstock. Combustible gases are directed through the nozzles and toward the tube as it is turned rotatably about its longitudinal axis and as the torch assembly is moved therealong to produce the hot zone. A temperature profile is produced across the hot zone which moves along on the surface of the tube, and, hence inside the tube, with a peak value sufficient to accomplish the desired reaction and deposition. The maximum temperature of the moving hot zone generally is within the confines of the torch assembly, although the temperature profile of the moving hot zone is not symmetrical with respect to the torch assembly.
During a deposition mode, the torch carriage moves slowly from the headstock of the lathe where dopants are moved into the glass tube to the tailstock where gases are exhausted. At the end of each pass from headstock to tailstock, the carriage is returned rapidly to the headstock for the beginning of another pass.
Subsequent to the deposition mode, the preform tube is collapsed into a rod-like member which is called a preform. It is this preform from which lightguide fiber is drawn. In the collapse mode, the torch is moved slowly from the tailstock to the headstock in the lathe and returned rapidly to the tailstock at the end of each pass.
As is known, one end portion of the doped preform tube is closed generally prior to the preform tube being collapsed. This is necessary in multimode fiber production, for example, in order to be able to pressurize the inside of the tube during the collapse mode in order to insure a circular cross-section. Obviously without the end of the tube being closed, the tube cannot be pressurized. For single mode fiber manufacture, a gas is passed through the tube during the first three or four passes of the collapse mode to reduce the moisture level, but even there it is necessary subsequently to close an end portion of the tube.
The closing of the tube is presently accomplished by allowing the torch assembly to dwell at the desired location until the end portion of the preform tube is caused to shrink and become closed. This process consumes in excess of ten minutes and at times results in the closed-off portion of the tube being oval in cross-section and offset from the centerline of the lathe between the headstock and the tailstock. Moreover, that cross-section, which is undesirable for drawing, seemingly is propagated along the length of the tube. It has been found that with the present method of closing, the preform cross-section in some instances is oval for about one-third of the length from the closed area to the headstock end of the tube.
Also, it has been found that bubble entrapment, which may occur adjacent to the end of the preform tube during closing, continues to exist after closing and perhaps even be propagated along the length of the tube. The portion of the tube in which the bubble is located is not collapsed, thereby decreasing the length of useable preform.
In U.S. Pat No. 4,486,214, there is disclosed a technique for collapsing a preform tube. As the preform tube is rotated, forces are applied with a roller to successive increments of length during exposure to and in a fixed relation to the zone of heat in each of a plurality of passes over a period of time. The distance at which the roller is moved toward the central longitudinal axis of the tube is increased at the beginning of each successive pass. The roller, which is cooled by causing contact between its external surface and a cooling medium, is not adaptable to be used to close the end portion of the tube adjacent to the tailstock. Inasmuch as the roller is located to one side of the torch assembly, the point at which the tube may be closed by the roller is outside the confines of the torch assembly where it is too cool to cause the tube to be closed in a relatively short period of time.
Clearly, there is a need for methods and apparatus for providing a closed portion of the preform tube prior to collapse without consuming too much time. This need has not been satisfied by the prior art technique of using a torch which requires undue time, and which may result in an unuseable cross-section of the preform tube along a substantial portion of its length.